diff --git a/src/pow.cc b/src/pow.cc
index 2c9d8c7..993b73b 100644
--- a/src/pow.cc
+++ b/src/pow.cc
@@ -6,37 +6,62 @@
 #define __STDC_LIMIT_MACROS
 #include <stdint.h>
 #include <string.h>
+#include <pthread.h>
 #include <arpa/inet.h>
 #include <openssl/sha.h>
 
 #define HASH_SIZE 64
-#define NTOHLL(x) ( ( (uint64_t)(ntohl( (unsigned int)((x << 32) >> 32) )) << 32) | ntohl( ((unsigned int)(x >> 32)) ) )
+#define NTOHLL(x)                                                  \
+  (                                                                \
+    ((uint64_t)(ntohl( (unsigned int)((x << 32) >> 32) )) << 32) | \
+    ntohl( ((unsigned int)(x >> 32)) )                             \
+  )
+#define SET_RESULT(res, nonce)          \
+  {                                     \
+    pthread_mutex_lock(&g_mutex);       \
+    if (g_result == RESULT_NOT_READY) { \
+      g_result = res;                   \
+      g_nonce = nonce;                  \
+    }                                   \
+    pthread_mutex_unlock(&g_mutex);     \
+  }
 
-int pow(size_t pool_size,
-        uint64_t target,
-        const uint8_t* initial_hash,
-        uint64_t max_nonce,
-        uint64_t* nonce) {
+typedef enum {
+  RESULT_NOT_READY,
+  RESULT_OK,
+  RESULT_OVERFLOW,
+  RESULT_ERROR
+} Result;
+
+// Global arguments.
+size_t g_pool_size;
+uint64_t g_target;
+uint8_t* g_initial_hash;
+uint64_t g_max_nonce;
+
+// Shared variables for threads.
+pthread_mutex_t g_mutex;
+Result g_result = RESULT_NOT_READY;
+uint64_t g_nonce;
+
+void* pow_thread(void* num) {
+  uint64_t i = *((size_t *)num);
   uint8_t message[HASH_SIZE+sizeof(uint64_t)];
   uint8_t digest[HASH_SIZE];
   uint64_t* be_nonce;
   uint64_t* be_trial;
-  uint64_t i;
   SHA512_CTX sha;
 
-  if (!max_nonce) {
-    max_nonce = UINT64_MAX;
-  }
-  memcpy(message+sizeof(uint64_t), initial_hash, HASH_SIZE);
+  memcpy(message+sizeof(uint64_t), g_initial_hash, HASH_SIZE);
   be_nonce = (uint64_t *)message;
   be_trial = (uint64_t *)digest;
-  i = 0;
 
-  while (1) {
+  while (g_result == RESULT_NOT_READY) {
     // This is very unlikely to be ever happen but it's better to be
     // sure anyway.
-    if (i > max_nonce) {
-      return -1;
+    if (i > g_max_nonce) {
+      SET_RESULT(RESULT_OVERFLOW, 0);
+      return NULL;
     }
     *be_nonce = NTOHLL(i);
     SHA512_Init(&sha);
@@ -45,12 +70,57 @@ int pow(size_t pool_size,
     SHA512_Init(&sha);
     SHA512_Update(&sha, digest, HASH_SIZE);
     SHA512_Final(digest, &sha);
-    if (NTOHLL(*be_trial) <= target) {
+    if (NTOHLL(*be_trial) <= g_target) {
+      SET_RESULT(RESULT_OK, i);
+      return NULL;
+    }
+    i += g_pool_size;
+  }
+  return NULL;
+}
+
+int pow(size_t pool_size,
+        uint64_t target,
+        const uint8_t* initial_hash,
+        uint64_t max_nonce,
+        uint64_t* nonce) {
+  g_pool_size = pool_size;
+  g_target = target;
+  g_initial_hash = (uint8_t *)initial_hash;
+  g_max_nonce = max_nonce ? max_nonce : UINT64_MAX;
+
+  pthread_mutex_init(&g_mutex, NULL);
+  pthread_t threads[pool_size];
+  size_t args[pool_size];
+  size_t i;
+  int error;
+
+  for (i = 0; i < pool_size; i++) {
+    args[i] = i;
+    error = pthread_create(&threads[i], NULL, pow_thread, &args[i]);
+    if (error) {
+      SET_RESULT(RESULT_ERROR, 0);
       break;
     }
-    i++;
   }
 
-  *nonce = i;
-  return 0;
+  // Wait for only spawned threads.
+  while (i--) {
+    pthread_join(threads[i], NULL);
+  }
+
+  switch (g_result) {
+    case RESULT_OK:
+      *nonce = g_nonce;
+      error = 0;
+      break;
+    case RESULT_OVERFLOW:
+      error = -1;
+      break;
+    default:
+      error = -2;
+  }
+
+  pthread_mutex_destroy(&g_mutex);
+  return error;
 }
diff --git a/src/worker.cc b/src/worker.cc
index 9a29a6a..91151ba 100644
--- a/src/worker.cc
+++ b/src/worker.cc
@@ -43,7 +43,12 @@ class PowWorker : public NanAsyncWorker {
   void HandleOKCallback () {
     NanScope();
     if (error) {
-      Local<Value> argv[] = {NanError("Max safe integer overflow")};
+      Local<Value> argv[1];
+      if (error == -1) {
+        argv[0] = NanError("Max safe integer overflow");
+      } else {
+        argv[0] = NanError("Internal error");
+      }
       callback->Call(1, argv);
     } else {
       Local<Value> argv[] = {NanNull(), NanNew<Integer>(nonce)};
@@ -70,7 +75,7 @@ NAN_METHOD(PowAsync) {
   uint8_t* initial_hash = (uint8_t *)malloc(length);
 
   if (initial_hash == NULL) {
-    Local<Value> argv[] = {NanError("Cannot allocate memory")};
+    Local<Value> argv[] = {NanError("Internal error")};
     callback->Call(1, argv);
   } else {
     memcpy(initial_hash, buf, length);